Not sure about mssql, but I was doing some testing, and was getting similar errors. After a reboot or two, and more tinkering it eventually found the old backups and restored using the reinstalled UrBackup Client.
I was testing by:
Snapshot hyper-v machine
install UrBackup client and run backup
restore to snapshot
Make sure UrBackup Client connected to server (using the “download client function” in the UrBackup Server web interface)
try to restore using server interface or right-click systray and Access/Restore Files.